During the past year our efforts have been devoted to a continuing examination of the regulation of several neurotransmitter receptors. One study has utilized selective neuronal lesions to work out the localization of histamine H-1, beta-adrenergic, GABA and benzodiazepine receptors. Other studies have clarified the organ and species variations in properties of histamine H-1 receptors. Considerable exploration of dopamine receptors has provided new ligands 3H-ADTN. Lesion studies have shown two physically distinct dopamine receptors only one of which is regulated by guanine nucleotides. Two separate populations of alpha-adrenergic receptors have been demonstrated in the brain and periphery.